distutils-r1.eclass: make *_install_for_testing install proper egg-info

Currently distutils_install_for_testing does not install the complete
egg-info into ${TEST_DIR}. This was first noticed by W. Trevor King
and reported in bug #524322. Based on info found in the related upstream
setuptools bug I added the necessary call to setuptools to create
the complete egg_info.

Without this certain packages (like cryptography) fail during testing
because they use introspection but the metadata isn't available.

Bug: https://bugs.gentoo.org/524322

eclass/distutils-r1.eclass

@@ -288,6 +288,8 @@ distutils_install_for_testing() {
 	PYTHONPATH=${libdir}:${PYTHONPATH}
 
 	local add_args=(
+		egg_info
+			--egg-base="${libdir}"
 		install
 			--home="${TEST_DIR}"
 			--install-lib="${libdir}"